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Interpretive Summary: Cercospora leaf spot is a destructive fungal disease of beet and occurs in most beet production areas. One important method of managing the disease is the use of varieties with resistance to the pathogen, Cercospora beticola. In continuing efforts to identify sources of resistance to Cercospora, 30 Plant Introdcutions (PIs) from the USDA-ARS National Plant Germplasm System were tested for their response to the leaf-spotting fungus, Cercospora beticola. An additional three entries resulting from crossing of previous years (2011-2014) PIs selected from Cercospora leaf spot tests with USDA germplasm were included for evaluation. Beets were planted on May 6 and inoculated on July 5. The disease was severe enough to start rating by mid August. Significant variability was detected in disease severity between different entries. One PI was not significantly different from the resistant control at the first rating date, but all entries had significantly higher disease severity than the resistant control at all later rating dates. Twenty-one PIs and all three of the crossing entries had significantly lower disease levels than the susceptible control at the final rating date and five of these PIs, as well as two of the three EL entries also had significantly lower disease ratings than the susceptible control at the third rating date. Six of the PIs and one of the EL entries produceed seed stalks (i.e. were annual) during the study. PIs identified in these screenings show promise as sources for resistance breeding and expanding genetic diversity in this crop, and this is supported by the results of crosses with materials from previous years screening.

Technical Abstract: Thirty Plant Introductions (PIs) from the USDA-ARS National Plant Germplasm System (NPGS) Beta Collection were evaluated for resistance to Cercospora beticola in an artificially produced epiphytotic environment. In addition, three entries resulting from crossing of previous years (2011-2014) PIs with East Lansing germplasm were included in the test. Two entries were selected for their general levels of agronomic performance to augment genetic diversity in the cultivated germplasm pool and one, EL-A12-00029, was selected for Cercospora leaf spot performance. A randomized complete-block design, with three replications was used to evaluate germplasm at the Michigan State University Saginaw Valley Research and Extension Center (SVREC) near Frankenmuth, MI. The field had been planted in wheat with clover underseeded in 2015. Internal controls included a susceptible check, 12N0050, and a resistant check, EL50/2 (PI 664912). Single-row plots 4.5 m long, with 51 cm between rows were planted on May 6. The nursery was spray-inoculated on with a liquid spore suspension of C. beticola. Visual evaluations of the plot with a disease index (DI) on a scale from 0-10 where 0=no symptoms and 10=all leaves dead. Evaluations were made on four times, with the peak of the epidemic occurring around Sept. 6. An evaluation was attempted subsequently, but several PIs were losing leaves following production of seed stalks and others were showing new leaf growth following defoliation from Cercospora leaf spot, so these ratings were not used. The moderate night temperatures in the summer of 2016, combined with high humidity and limited rainfall, contributed to a moderate leaf spot epiphytotic. At the Sept 6 rating, means of the resistant and susceptible internal controls for the entire nursery (including three additional experiments) were 3.4 and 6.8, respectively, across the nursery. At the peak of the epiphytotic in 2015 (Sept. 9), these means were 3.2 and 7.0 for resistant and susceptible checks, respectively. Means of contributor lines in the entire nursery (including three additional tests) in 2016 ranged from 3.0 to 7.0. An analysis of variance on the disease indices determined that there were significant differences among entries (p<0.05) on all dates of evaluation. All accessions were significantly different from the resistant control at the final three rating dates, but one accession, PI 232893, was not significantly different from the resistant control at the first rating date. At the final rating date 21 accessions and all three East Lansing breeding lines were significantly different from the susceptible control. Five of these (PI 232893, PI 535822, PI 535827, PI 535832 and PI 583781) and two of the EL entries (789 and 791) also were significantly different from the susceptible control at the third rating date. Six PIs (PI 334063, PI 334064, PI 535822, PI 535825, PI564759, and PI 583780) and one EL entry (789) required removal of seed stalks from at least one replicate during the season. These data, and more information on the accessions evaluated, are available through the USDA-ARS GRIN database at http://www.ars-grin.gov/npgs.